Smallville and Mouse Guard, among others, track things like bad moods that a character may suffer - Angry, Tired, and the like. This works well for tracking consequences of conflicts, so that an argument or a physical challenge don't necessarily result in hit point damage or nothing. Ultimately, though, I think they're a little more granular and specific than I'm particularly interested in stealing for other games.
D&D 3.x introduced "new" damage tracks to the game, in the form of ability score damage and negative levels. These mechanics were interesting and new takes on simulationism when 3e first released, but over the course of several years and a great deal of design work (monsters, magic items, spells, even a revised edition... you name it) it became increasingly clear that they slow down play egregiously, because so much recalculation takes place. In my experience, players repeat the math process for each separate roll that involves a reduced ability score or negative level. It's the same kind of mental slowdown and "no wait that should have missed" that beneficial spell effects like bless and prayer cause, but with the added difficulty of lost spell slots, spell DCs, and spell access. In short, adjusting ability scores has cascading effects throughout the rules that can be all too easy to overlook.
I wonder if it would actually represent simplification to count ability score damage upward in the form of "damage" scores, in much the same way that negative levels count upward. For example, a character has 16 Strength and fails a saving throw against Medium Spider Venom (1d4 Str/1d4 Str). What if the result was 16 Strength and 4 points of a Weakened stat (-2 to attack, damage, and Strength saves)? This would make it so PCs don't lose feat access when they no longer meet feat prereqs (but then, that's almost always overlooked anyway), but still become helpless when their Weakened stat equals their relevant ability score. It would similarly not change spells prepared in any way, though there's no reason it couldn't apply to spell DCs. (But seriously, do yourself a favor and turn all spell DCs into attack rolls against fixed defenses so that that's actually intuitive.)
But this isn't quite on the mark of what I mean to cover in this post.
D&D 3.x introduced "new" damage tracks to the game, in the form of ability score damage and negative levels. These mechanics were interesting and new takes on simulationism when 3e first released, but over the course of several years and a great deal of design work (monsters, magic items, spells, even a revised edition... you name it) it became increasingly clear that they slow down play egregiously, because so much recalculation takes place. In my experience, players repeat the math process for each separate roll that involves a reduced ability score or negative level. It's the same kind of mental slowdown and "no wait that should have missed" that beneficial spell effects like bless and prayer cause, but with the added difficulty of lost spell slots, spell DCs, and spell access. In short, adjusting ability scores has cascading effects throughout the rules that can be all too easy to overlook.
I wonder if it would actually represent simplification to count ability score damage upward in the form of "damage" scores, in much the same way that negative levels count upward. For example, a character has 16 Strength and fails a saving throw against Medium Spider Venom (1d4 Str/1d4 Str). What if the result was 16 Strength and 4 points of a Weakened stat (-2 to attack, damage, and Strength saves)? This would make it so PCs don't lose feat access when they no longer meet feat prereqs (but then, that's almost always overlooked anyway), but still become helpless when their Weakened stat equals their relevant ability score. It would similarly not change spells prepared in any way, though there's no reason it couldn't apply to spell DCs. (But seriously, do yourself a favor and turn all spell DCs into attack rolls against fixed defenses so that that's actually intuitive.)
But this isn't quite on the mark of what I mean to cover in this post.